The present invention relates to data networks suitable for industrial control and in particular to gateways communicating between a spanning tree (ST) network and device level ring (DLR) network.
Networks used for communication among industrial controllers differ from standard networks in that they must operate to communicate data reliably and within predefined time limits among network devices that control equipment. A bounded response time may be provided by communication protocols that reserve network bandwidth and schedule messages. Network reliability may be provided by the introduction of redundant network components.
Many computer networks provide for automatic “repair” of the network in the event of network device failure by switching between redundant components. These protocols can take a relatively long time to reconnect the network (as much as 30 seconds) and thus are unacceptable for industrial control networks where the controlled process often cannot be undirected during this period without serious consequences.
High-speed correction for network failure in an industrial control environment can be obtained by connecting network devices in a device level ring (DLR) where the ring network topology presents redundant paths (along the ring in two opposite directions) between any two devices. Normally the ring is “open” at a supervisor device for all standard data and thus operates in a normal linear topology for most data messages. The supervisor may send out “beacon” frames in both directions on the ring on different ports which are received back at the opposite port to indicate the integrity of the ring. If the ring is broken by device or media failure outside of the supervisor, the supervisor rejoins the ends of the ring at the supervisor to restore a continuous linear topology with the ring now separated by the failed component rather than the supervisor. Changes in the state of operation of the supervisor from “separated” to “joined” may be transmitted to the other nodes using notification frames so that these nodes can rebuild their MAC address routing tables used to associate a port with a destination address.
The error detection time of such ring systems can be quite fast, limited principally by the transmission rate of the beacons (every several microseconds). This rate defines the maximum time before which an error is detected and the ring may be reconfigured.
It is often desired to connect a DLR network with other networks, for example, those associated with devices that do not require the benefits of the DLR network topology. Such networks may permit a more flexible device interconnection, facilitated by a “spanning tree protocol” (STP) that detects and eliminates possible “loops” in connections between devices, such loops which otherwise might permit messages to pass indefinitely in circles through the network. As is understood in the art, spanning tree protocols identify loops in a network built with infrastructure devices called bridges, and provide instructions to bridges to block certain ports to eliminate these network loops. These instructions are transmitted as “bridge protocol data units” (BPDUs) to the various bridges in the network.
The loop structure of a DLR can be incompatible with a spanning tree (ST) type network, which attempts to eliminate loops. This incompatibility may be accommodated when providing a gateway between a DLR and ST type network by ensuring that each given DLR network has only a single gateway. This limitation to the number of gateways, however, increases the risk that a single gateway failure will prevent communication between the two networks.